1. Field of the Invention
The present invention pertains to the field of power converters and utilizes a direct A.C. input to A.C. output conversion technique without an intervening D.C. conversion stage. This D.C. stage is responsible for the high total harmonic distortion, THD, and low power factors common to prior art. The control of THD and power factor correction are responsible for the high complexity of electronic ballasts and similar circuits driving high voltage A.C. devices.
2. Prior Art
In general, such apparatus have been designed to convert an A.C. voltage input to D.C. which in turn is converted to a high voltage, high frequency, A.C. source suitable for driving an A.C. receptive load. By their very nature, these circuits exhibit poor power factor and total harmonic distortion characteristics as well as lower electrical efficiency and reliability. Much of the complexity of previous inventions has been aimed at remedying these short comings at the expense of higher costs and lower reliability. It is well known in the art that reliability is inversely affected by the number of components.
A common power inverter application is to provide compatible A.C. power to operate fluorescent lamps. Fluorescent lamps are commonly used to provide illumination, particularly in industrial environments where their economy of power utilization is highly desirable. Because of their greater efficiency in converting electricity to light, the cost of utilization is significantly reduced when compared to incandescent lighting.
A common limitation of these devices has been that they have required sophisticated circuitry to control the above mentioned lamps. Most modem electronic fluorescent lamp ballasts utilize a D.C. to A.C. inverter circuit to strike and supply operating power to the lamps. As control circuitry is added to accomplish regulation or dimming of the light source, the complexity and cost has historically increased dramatically while the reliability and manufacturing consistency have decreased. Additionally the D.C. rectification and power storage stage interacts in an undesirable manner to cause high total harmonic distortion, THD, and a lower power factor thereby requiring further complexity to compensate for these defects.
The present invention addresses the above limitations in several ways. The first is that the apparatus described herein eliminates the prime cause of limited reliability, the power storage electrolytic capacitor. Secondly, the preponderance of existing electronic ballast circuits utilize a half bridge to control power delivered to the load. Because of its very nature, control is only on one half of the cycle, thereby limiting controllability. Additionally, the diode bridge is responsible for a lower efficiency, lower Power Factor, and Increased THD. By way of example, each diode has a voltage drop of 0.7 volts. In a 100 watt supply the diode bridge alone is responsible for a voltage drop of 2.8 volts which yields a loss of 2.8 watts or almost 3 percent lower efficiency. To achieve a high power factor the load should be resistive in nature. These effects are almost totally eliminated by the absence of the DC conversion stage from the design of the present invention.